Low back pain (LBP) is a complex condition that affects 65-85% of the population, and is the leading musculoskeletal condition contributing to disability in the United States. Seventy-five percent of individuals undergoing surgical and rehabilitative interventions for this condition experience suboptimal or poor outcomes. These patients demonstrate disability and deficits in functional capacity, including strength and endurance of the lumbar musculature. Muscle-specific changes in individuals with LBP include altered muscle volume, fatty infiltration and fibrosis, and fiber area and type. Importantly, these changes are insensitive to rehabilitation in patients with continued chronic or recurrent symptoms. While normal disuse-related atrophy in the presence of LBP is expected, more severe or chronic pathology, such as inflammation and fiber damage, may be inducing irreversible fiber degeneration and fatty/fibrotic tissue changes that impair muscle function and recovery. While the structural and adaptive capacities of healthy muscle are well understood, muscle responses to exercise in the presence of pathology are less clear. To address this gap in knowledge, the purpose of this proposal is to use an innovative imaging tool, intravoxel incoherent motion imaging (IVIM), to investigate perfusion-related changes with muscle contraction in common rehabilitation exercise protocols in individuals with and without chronic LBP. This tool allows for us to investigate muscle activation in 3 dimensions, and accounts for muscle tissue compositional changes with disease. Our central hypothesis is that the lumbar paraspinal muscles are used differently in individuals with and without chronic LBP. Specific Aim 1 will use IVIM MRI to compare muscle perfusion patterns across exercise modalities between people with and without LBP. Specific Aim 2 will use the addition of electrical stimulation to investigate activation-related deficits in individuals with varying levels of muscle fatty degeneration. Understanding how muscles respond to exercise when they are compositionally altered, and if this compositional change is a result of voluntary activation deficits, will provide novel information about the 3 dimensional nature of muscle recruitment during commonly used rehabilitation exercises. It will also provide key information on the potential mechanisms for muscle- related activation deficits in individuals with LBP and how these are altered in diseased muscle tissue. These experiments will elucidate the structural, mechanical, and adaptive potential of lumbar spine muscles in these patients. This contribution is significant because it is the first step in a precision medicine approach aimed at establishing appropriate and targeted exercise prescriptions for reversing atrophic or degenerative muscle changes that obstruct patient recovery. The proposal is innovative because it utilizes novel MRI methods to measure muscle perfusion as a proxy for muscle activation and adaptation with exercise. As such, we expect to impact rehabilitation by inducing tissue-level change in LBP.